I found these spares, which were for my (circa 1993) Bombora 283. There’s a powerbox-style fin box, alloy top-plate and pop rivets (for the fin box), and a mast track. The mast track secures with 5 bolts; I believe that some versions used 3. The alloy plate has some stress marks; the other parts are unused.
These parts also fit the 270 and the 295, and probably the X-it (not sure about this).
If you have a use for all or any of these, PM me, and I will post them to you.